Our goal is to understand how a successful vaccine induces long-term immunological memory and protective immunity in humans. To achieve this goal we have initiated a detailed cellular and molecular characterization of human immune responses induced by the yellow fever virus (YFV-17D) vaccine. This is one of our most efficacious vaccines and induces long-term immunity that lasts for decades. Also, since YFV-17D is a live attenuated vaccine and most of the U.S. population is not exposed to YFV-, this provides a unique opportunity to analyze antiviral responses in humans during the course of a primary infection and then to monitor the generation and maintenance of immune memory after resolution of the infection. One of the potential benefits of understanding how a successful vaccine induces long-term protective immunity is that this knowledge can be applied to improving other less effective vaccines and, more importantly, to develop new vaccines against emerging diseases. During the previous cycle of funding we have made substantial progress in characterizing human memory T and B cell responses not only to YFV but also after immunization with small pox and influenza vaccines. In this renewal application we will focus our studies on CDS T cells and examine the mechanisms that regulate human effector and memory CDS T cell differentiation. The following specific aims are proposed to achieve our goals: 1) To identify transcription factors that regulate naive to effector CDS Tee// differentiation. 2) To analyze the in vivo turnover of human YFV specific CDS T cells and to examine their homing potential. 3) To define the genomic and epigenetic changes that occur during human memory CDS T cell differentiation.These studies will will provide the first view of the transcriptional changes that occur following CDS T cell differentiation in humans and will provide unique markers that will enable identification, isolation, and characterization of the differentiated cell subsets. Examination of the epigenetic DMA methylation marks during the progression of the T cell response, as well as between CD8 T cells responding to acute versus chronic viral infections will provide a potential mechanistic view of how memory CDS T cell differentiation is globally regulated.